Magnetoresistive random access memories (MRAMs) combine magnetic components with standard silicon-based microelectronics to achieve non-volatility, high-speed operation, and excellent read/write endurance. In an MRAM, information is stored in the magnetization directions of the free magnetic layers of the individual bits in the memory array. In a standard MRAM, the bit state is programmed to a “1” or “0” using applied magnetic fields generated by currents flowing along adjacent conductors—e.g., orthogonally-situated word lines and bit lines. The applied magnetic fields selectively switch the magnetic moment direction as needed to program the bit state.
In spin-transfer MRAM devices, however, the bits are written by forcing a current directly through the stack of materials that make up the bit. Generally speaking, the write current, which is spin polarized by passing through one layer, exerts a spin torque on the subsequent layer. This torque can be used to switch the magnetization of the free layer between two stable states by changing the write current polarity. Spin-transfer MRAMs are advantageous in that they may provide for greater density with lower power consumption.
In order to reduce write current, some spin-transfer MRAMs incorporate a dual spin-filter, in which the bit stack includes two different spin-polarizing layers—one on each side of the free layer—to improve spin-transfer efficiency by increasing the effective spin-transfer torque on the free layer. Referring to FIG. 1, for example, a typical spin-transfer MRAM 100 with a dual spin-filter generally includes a free magnet layer 110 separated from a top spin polarizer 130 and a bottom spin polarizer 132 by nonmagnetic spacers 108 and 112. The top spin polarizer includes a fixed magnet layer 106 and an antiferromagnet layer 104 which acts to “pin” fixed magnet layer 106 to a particular alignment via exchange coupling. The bottom spin polarizer includes two fixed magnets 114 and 118 separated by a non-magnet layer 116 and an antiferromagnet 120 used to pin fixed magnet 118 to a particular alignment. An antiferromagnetic coupling across nonmagnet 116 aligns fixed magnets 114 and 118 antiparallel to each other. In this illustration, bold arrows are used to indicate the direction of the magnetic moment for individual layers. The magnetization directions of the top and bottom spin polarizers 130 and 132 are set using a high-temperature anneal in an applied magnetic field pointing to the right in FIG. 1. Each of the nonmagnetic spacers 108 and 112 can be either an electrically insulating tunnel barrier or an electrically conductive metallic layer. In reading the state of the free magnet 110, the output signal is generated from the combined magnetoresistance signals across both of the nonmagnetic spacers 108 and 112. The magnetoresistance signal is due to tunneling magnetoresistance if the spacer is an electrical insulator, or to giant magnetoresistance if the spacer is a nonmagnetic metal.
When a write current 102 (IDC) flows through structure 100 from bottom to top (wherein arrow 102 is intended to show the direction of electron flow), electrons passing through fixed magnet 114 are spin-polarized to the left (in this illustration) and therefore place a torque on the free magnet 110 to switch its moment to the left. As electrons cross free magnet 110 and are incident on magnet 106 of top spin polarizer 130, some electrons will reflect back to free magnet 110 with a spin-polarization to the left, thus also placing a torque to switch free magnet 110 to the left. Thus, the torques from the top and bottom spin polarizers combine in an efficient manner. Free magnet 110 can similarly be switched to the right by forcing electron flow in the opposite direction. In the interest of high spin-transfer efficiency and low write current, the magnets neighboring free magnet 110 (i.e., fixed magnet 106 and 114) are preferably aligned antiparallel to each other.
Known dual spin-filter spin-transfer MRAMs are unsatisfactory in a number of respects. For example, top antiferromagnet 104 typically consists of a relatively thick layer (e.g., 100-200 Angstroms) of PtMn or IrMn. During fabrication, the thin film bit stack 100 must be etched to define the individual memory bits in the MRAM array. The presence of the relatively thick antiferromagnet layer 104 significantly increases etch complexity.
To address this problem, other prior art MRAMs are fabricated without a top antiferromagnet layer, as shown in FIG. 2. In this design, fixed magnet 106 is set by applying a strong magnetic field (pointing to the right in FIG. 2) during annealing, which also sets bottom spin polarizer 132 as shown. However, while this design improves manufacturability by removing the topmost antiferromagnet layer, the resulting structure is still undesirable in that a large offset magnetic field 202 affects free layer 110 due to the presence of top fixed magnet 106. The offset field can disrupt the symmetry between the magnitude of the write currents needed for switching the free layer 110 in either of its two directions, and can decrease the reproducibility of spin-transfer switching between different bits in the memory. In comparison, the fields from the bottom fixed magnets 114 and 118 nearly cancel each other, assuming the moments of magnets 114 and 118 are closely balanced.
It is therefore desirable to provide dual spin-filter spin-transfer MRAMs with improved manufacturability while maintaining a low offset magnetic field at the free magnet layer. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.